Heating, ventilation and air-conditioning (HVAC) systems have to satisfy three primary comfort functions, namely heating and cooling, humidity control, and ventilation. These functions are usually delegated to either a central forced convection air-conditioning system, or unitary air-conditioners, or hydraulic heating/cooling systems (like convective fan-coils or radiant panels). HVAC systems are typically comprised of fans and ductwork for moving air where needed. An HVAC system will include a cooling and heating section for cooling and heating the air. In most HVAC systems, air is drawn in, filtered, cooled and dehumidified or heated and humidified, and then delivered to a room. The greatest portion of this air is drawn from the conditioned space for recirculation through the HVAC system.
The HVAC industry is economically sensitive to costs. Because of the competitive nature of both the construction and HVAC industries, HVAC systems must be inexpensive to install. Of a more global interest though, is the cost to operate and maintain HVAC systems. Often, a building owner will replace an aging HVAC system as the reduction in operating and maintenance costs can offset the retrofit cost, sometimes in a matter of months. Broad social and energy policies also favor more efficient HVAC systems. In these days of electricity deregulation and conservation, it has become even more important to conserve energy consumption.